Re: [CAD_CAM_EDM_DRO] ECM Revisited

Graham,

Very interesting. Keep us updated on the progress and tell us how long did
it take to get the the 250 mm mark. It's surely worth it even if it takes a
whole day to get to that depth. Making small diameter long and straight
holes is always a pain in any metal workshop. If it can drill through
tungsten carbide at the speed you described then it sure is faster than my
old sinking edm.

Zafar

----- Original Message -----
From: Graham Stabler <eexgs@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 30, 2005 7:24 AM
Subject: [CAD_CAM_EDM_DRO] ECM Revisited


> Some may remember a little experiment I did with Electro Chemical
> Machining a year or two ago:
>
> http://www.indoor.flyer.co.uk/ecm.htm
>
> The idea is you take a DC power supply some salt water and an
> electrode and you make holes or dies, much like EDM.
>
> Recently in the department they have been looking at machining a
> motor casing in aluminium. This requires 240mm long holes at 5mm
> diameter. These are pretty long holes. We have a sinker EDM but
> the the travel is not nearly enough. I suggested a homebrew ECM
> setup, well they were unconvinced to say the least so I had a go in
> my lunch breaks.
>
> Firstly you can do some clever tricks with ECM because of the way it
> works. If you want to make long holes you can either use a long
> metal tube as an electrode with insulated sides and control the
> height using EDM type anti-short circuit methods (read complex). Or
> better use a glass tube with a wire inside. The glass tube is the
> insulator and also carries the salt water. You can pretty much let
> the glass tube sit against the workpiece and just let the electrode
> wire stop just short of the end of the tube. Add a simple gravity
> feed and you have a hole driller.
>
> This is what I did. First thing I learnt is that it would be better
> to have a metal tube within the glass tube, it would put the
> electrode closer to the edges of the glass tube and this should mean
> that the progress of the glass tube through the material is speeded
> up as usually the point where the glass tube hits the side of the
> hole being machined is the point that machines the slowest as it is
> furthest away from the electrode. This would also mean you could
> pass the fluid down the metal tube and use the glass tube as a
> sleeve. The second thing I learnt is that as the tube enters the
> aluminium the tiny pump I am using really starts to struggle as it
> pushes the fluid up the tiny gap.
>
> My solution has been a new electrode design, I started with a EDM
> sinker electrode, this is a brass tube but it has three smaller
> brass tubes running up it, in my case I chose it as is is nice and
> stiff and also straight. I then soldered a small copper washer to
> the end of the tube ( actually I soldered a bit of sheet to it and
> turned it in the lathe afterwards). This washer is larger than the
> tube and defines the size of the hole, well nearly. To stop the
> washer shorting with the workpiece a small plastic part was made,
> this is basically just a little tube that extends past the washer to
> prevent it bottoming out and also prevents shorts with the side
> walls. It glues securely to the tube. The sides of the tube are
> then laquered to prevent any ECM action on the sides of the hole as
> it is cut.
>
> The nice thing about this electrode design is that the insulating
> tube can be very thin so that it does not impede the machining speed
> too much, whats more because of the washer style electrode being
> wider than the tube the flow is restricted over a much shorter
> distance, this means that my windsreen washer pump can actually keep
> working.
>
> The rig at the moment is just a basic slide, the weight of the
> carriage provides the gravity feed. The power supply is operating
> from 20-60v upto about 5 amps. Everything gets a bit warm and this
> caused problems when the plastic insulator came unglued, I have just
> reglued it (with decent epoxy) and will try again tommorow. I had
> probably made about 3mm progress in 5 min, not light speed perhaps
> but pleasing.
>
> A further idea I had is to return to glass tube and creat a slight
> bell mouth in order to reduce flow restriction and also have a short
> length of tube as the electrode inside the tube.
>
> Sorry for the long and random post but this is exciting to me, as
> you can ECM hardened materials as well as aluminium it is pretty
> handy and very easy to do in theory. I suspect with a few brains
> working on it something very capable could be knocked up for pennies.
>
> Graham
>
>
>
>
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